Technical Field
The present invention relates to curable compositions for preparing transparent antistatic hard coatings, articles exhibiting at the same time good antistatic properties and good abrasion properties coated therewith, in particular optical and ophthalmic lenses for eyeglasses, and a process to prepare such articles. These inventions are based on the use of additives to increase antistatic properties without lowering abrasion and scratch-resistance properties.
Description of Related Art
It is well known that optical articles, which are essentially composed of insulating materials, have a tendency to get charged with static electricity. The charges which are present at the surface of said optical articles create an electrostatic field capable of attracting and fixing light objects moving near said charged surface, generally small size particles such as dusts.
In order to decrease or suppress attraction of the particles, it is necessary to decrease the intensity of the electrostatic field, i.e. to decrease the number of static charges which are present at the surface of the article. This may be carried out by imparting mobility to the charges, for instance by introducing in the optical article a layer of a material inducing a high mobility of the charges. Said layer is called antistatic layer.
By “antistatic”, it is meant the property of not retaining and/or developing an appreciable electrostatic charge. An article is generally considered to have acceptable antistatic properties when it does not attract or fix dust or small particles after one of its surfaces has been rubbed with an appropriate cloth. It is capable of quickly dissipating accumulated electrostatic charges.
The ability of an optical article to evacuate a static charge can be quantified by measuring the time required for said charge to be dissipated (charge decay time). Thus, antistatic optical articles have a discharge time in the order of 100-200 milliseconds, while static optical articles have a discharge time in the order of several tens seconds, sometimes even several minutes. A static optical article having just been rubbed can thus attract surrounding dusts as long as it requires time to get discharged.
Numerous materials are well known to confer antistatic properties, but few present the transparency required for optical applications, i.e. transmittance of visible light higher than 90%.
Transparent antistatic coatings may be obtained by vapor deposition of metals or metal-oxides like indium tin oxide or vanadium oxide under. But these coatings are fragile and do not resist harsh mechanical conditions like bending or thermal stress.
Coatings applied by wet route, i.e. by applying a flowable composition to be cured have also been investigated. Antistatic properties in such coatings are generally obtained by incorporation of specific additives into an aqueous based composition, for instance a hard coating composition.
Conductive polymers represent the most used additives in wet coatings. These polymers can build a percolating network allowing for charge dissipation. Poly(3,4-ethylenedioxythiophene) (PEDOT) is especially interesting as it is transparent. Initially used in organic resins and films, this polymer has been modified to be added in aqueous compositions, which are desirable for environmental and health reasons. For instance, PEDOT is commonly grafted with Poly Styrene Sulfonate (PSS), allowing for good dispersion in water. To further enhance conductivity, various additives may be used like sulfonate surfactant (US 2011/248223). However, formulation of high molecular weight polymers in Sol-Gel coatings is difficult, as they may aggregate and have negative effect on transparency.
Another method to bring electrical conductivity in aqueous based compositions consists in adding as specific additives very light charge holders like positively charged lithium cations.
Further improvements have been proposed by addition of ionic liquids (JP2008/184556).
However, the addition of such specific additives into aqueous based compositions, and more particularly hard coating compositions is always detrimental to the abrasion properties of coating obtained upon curing of such modified compositions.
Finally, there is still a need to improve antistatic properties of hard coatings for optical articles, without lowering on one hand transparency and, on the other hand, mechanical properties such as abrasion resistance, scratch resistance, mar resistance, etc.